Modify configurations for APC to work
Let's edit our PHP configuration file to enable APC. My PHP configuration file is located at /etc/php5/cgi/php.ini.

$ sudo vi /etc/php5/cgi/php.ini
Add the following line in the very end of the configuration file.

extension=apc.so
If you just leave it the way it is APC will be using the default values for its configurations, which should do for any average user. You can overwrite any configuration key-value pairs here if you so wish. To see the full list of the configuration keys and accepted values go to http://php.net/manual/en/apc.configuration.php.